Current cellular communication systems according to 3GPP specifications are typically based on the Evolved Packet System (EPS) which provides a new radio interface and new core network functions for broadband wireless data access. The EPS radio interface consists of (LTE/LTE-A-based) packet-switched network access via E-UTRAN, which is provided in addition to (2G/3G-based) network accesses via UTRAN and GERAN.
An UE representing a subscriber can be camped on, and thus served by, any one of different network accesses or domains, namely E-UTRAN or UTRAN or GERAN, but not two network accesses or domains simultaneously. That is, when the UE is camped on E-UTRAN, it is not reachable over UTRAN or GERAN.
In an effort to ensure consistent service experience for subscribers in the EPS framework, various concepts are proposed for implementation in existing CS network accesses or domains.
On the one hand, the concepts of Circuit-Switched Fallback (CSFB) and Single Radio Voice Call Continuity (SR-VCC) are specified, which are applicable in the EPS framework. By the CSFB and SR-VCC concepts, specific services can be continued by way of the circuit-switched network access or domain, i.e. via UTRAN or GERAN, when these can not or no longer be provided by way of the packet-switched network access or domain, i.e. via E-UTRAN. For implementing these concepts, no specific control of radio resources is required, and thus a correspondingly configured network node with respective functionality can be connected to MSC servers in existing CS network accesses or domains.
On the other hand, the concept of IMS Centralized Service (ICS) is specified, which is applicable in the EPS framework. By the ICS concept, the services are provided centrally from the centralized IMS service domain, instead of being provided locally from a specific network access or domain. The centralized IMS service domain is access agnostic. For implementing this concept, an appropriate control of radio resources is required, since the centralized IMS service domain has to ensure that the UE is serviceable (i.e. able to connect to or be accessible via a circuit-switched radio access network). Hence, a correspondingly configured network node with respective functionality, such as an ICS-enabled MSC server, is to be deployed in existing CS network accesses or domains.
FIG. 1 shows a schematic diagram illustrating the ICS architecture according to 3GPP TS 23.292, which represents an architecture of a centralized service network system.
In the ICS architecture (also referred to as IMS service continuation and continuity reference architecture), the circuit-switched (CS) domain (including GERAN/UTRAN, MSC Server, etc.) becomes a simple CS access network which does not execute services itself, but only ensures that the UE sessions are delivered to the centralized IMS service domain where the services are actually executed. In both cases of mobile-terminating and mobile-originating sessions, the centralized IMS service domain ensures that the UE is serviceable. When an UE using ICS roams between the different access networks (like E-UTRAN and CS), the UE can use the same services independently from its currently used access because those are executed in the centralized IMS service domain.
When the UE is under CS access and performs a usual location update procedure to the MSC Server, the MSC Server registers itself in the IMS service domain, namely in the Service Centralization and Continuity Application Server (SCC-AS) therein, as a contact point for the UE for the provision/execution of centralized services. So, when a mobile-terminating session arrives at the SCC-AS for this UE, the SCC-AS will know to which MSC Server the session should be routed in order to reach the UE so as to provide/execute the requested service.
Accordingly, the service provision/execution for a UE in such centralized service network system, like the ICS service domain, is based on the registration of a mobile switching system, such as a MSC server, as a contact point for centralized services for the UE. In order to ensure service continuity in such centralized service network system, like the ICS service domain, it is thus required to ensure a correct and continuous registration of the contact point for centralized services for the UE.
For the implementation of any one of the aforementioned concepts for ensuring consistent service experience for subscribers in the EPS framework, correspondingly configured network nodes, i.e. MSC servers, with respective functionality are to be deployed in existing CS network accesses or domains. However, in view of the associated efforts and costs as well as potential reliability considerations or the like, operators do not want to replace or upgrade all existing MSC servers in their communication infrastructure at once so as to support the respective functionality. Rather, operators want to introduce such concepts by way of an overlay implementation, i.e. by replacing or upgrading only a part of the existing MSC servers in their communication infrastructure at once so as to support the respective functionality. That is, an overlay implementation is to be enabled/realized such that no changes are required in legacy network nodes (i.e. network nodes without the newly introduced CSFB, SR-VCC, ICS functionality), or the required changes are minimal.